In conventional surface wellheads, the surface casing hanger has a shoulder adapted to be landed on a seat in the bore of the surface wellhead, and is then sealed to close the annulus about casing string. For shallow wells, many casing strings are not tensioned, and the casing hanger is simply hung off this shoulder. To provide the desired length of the casing relative to the shoulder, an adjustment mechanism may be provided which allows for adjusting axially several inches the spacing between the engaging surface on the hanger and the shoulder on the wellhead for space-out.
As long as the depth of the water is not too great, the desired space-out of the casing hanger and the wellhead shoulder may be predicted with reasonable accuracy. However, as wells are drilled to considerably greater depths, the operator is faced with the problem of applying too little tension to the casing string once the hanger and thus the casing supported on the hanger is landed on the casing shoulder within the wellhead.
As a related problem, the operator preferably desires a certain tension on the casing once hung off from the shoulder on the wellhead. As water depths increase, it becomes more difficult to calculate the correct space-out, and even if properly calculated, that space-out requires for a deep well a long adjustment mechanism in the casing hanger for engagement with the shoulder. When the casing is hung off and the adjustment member is “fully stroked, ” the operator still may not have the desired tension in the casing hanger. Since the adjustment mechanism cannot pass below the shoulder on the wellhead, the operator is forced to either (a) accept the limited tension in the casing that is obtained, or (b) provide a different space-out along the length of the casing, or (c) select a wellhead and an adjustment mechanism for the casing hanger which allows for an even longer stroke length.
In systems where vertical adjustment of the casing relative to the wellhead are made, extremely long wellheads or hanger systems are required to accept the length of adjustment. Systems of this design result in substantially increased stack up height, which then requires more space between deck levels on offshore facilities.
Providing a plurality of vertically spaced seats of varying diameters on which the casing hanger shoulder may be landed, depending on the desired casing stretch, is not a satisfactory solution. Varying diameter seats would increase the radius of the bore through the wellhead housing in which the hanger is landed, thus increasing the I.D. of the wellhead equipment thereabove, which would greatly add to the expense of overall equipment. Also, seals would then have to be provided for sealing with varying diameter bores in the wellhead, and hangers with different diameter shoulders would have to be kept in inventory for landing on the different sized seats.
Some prior art systems contain a seal between two tubular members, so that the seal slides for the length of the desired adjustment. Split latch rings or threads may be used to mechanically interconnect the two tubular members. This sliding seal provides a substantial leak path in the casing hanger system, and the latch rings or threads reduce fatigue life due to dynamic loading.
The disadvantage of the prior art is overcome by the present invention, in an improved wellhead for supporting a hanger is hereinafter disclosed, wherein the tubular, such as casing or liner, supported on the wellhead may be selectively positioned with respect to the wellhead so that the operator may apply a proper tension to the string. An improved wellhead and hanger system accordingly is hereinafter disclosed for reliably supporting a tubular string with a desired stretch in the string.